Sure. But there are many ways of meeting your requirement, and I don’t know which one(s) you want.
For example, you could limit what the user could select in the Palette. In the main Diagram’s “ExternalObjectsDropped” DiagramEvent listener you could detect that a “Start” node had just been dropped, and then you could delete the “Start” node from the Palette. (By the way, you might be interested in a new sample: Two Diagrams)
Alternatively you could leave the Palette alone and decide in your “ExternalObjectsDropped” DiagramEvent listener that any “Start” nodes that were just dropped should be deleted if there already was a “Start” node in the diagram (i.e. not selected).
I think if I spent more time on this I could devise alternatives to these two scenarios. You need to decide what ramifications you want or can live with.